A need to protect integrated circuits against electrostatic discharges is known. These electrostatic discharges occur in particular when the integrated circuits are not under power, for example when they are being manually handled. The discharges occur between two external pins. This is why protection devices are usually provided. They are placed on each input/output pin of the integrated circuit, and enable both the clamping of the voltage and the flow of the charges without destroying the integrated circuit or allowing the integrated circuit to be damaged.
A prior art protection device as shown in FIG. 1 comprises, on each input/output pin of the integrated circuit, two series-connected diodes D1 and D2 as shown on the pin P1 in the example. One of the diodes, D1, is between an internal point N1 of connection to the pin and an internal positive power supply line A. The other diode, D2, is between an internal negative or zero power supply line B and the same internal connection point N1.
If the power supply potentials applied to the lines A and B are respectively Vcc, between 3 and 5 volts for example, and Vss, equal to zero volts, and if Vd1 and Vd2 denote the threshold voltage of the diodes D1 and D2, the voltages applied to the pin P are limited by the protection device to Vcc+Vd1 in positive terms and Vss-Vd2 in negative terms. This limiting, while it is satisfactory in the context of protection against electrostatic discharges, may prove to be highly inconvenient in operation.
Indeed, this limiting also takes place in an operational mode. In certain applications, the voltage applied in the operational mode to certain input/output pins of an integrated circuit may go beyond the level of a power supply voltage. An exemplary application in which voltage levels going beyond the level of a power supply voltage are used operationally, relates to the field of radio frequencies.
In certain radio-frequency applications, a reference signal is used in reception with respect to a virtual ground. For reasons of coupling capacitance, this virtual ground may be chosen in particular to be equal to the level of the positive supply voltage, Vcc in the example. A signal is then received at a pin of the integrated circuit. The DC value of this signal is linked to the level of the positive supply voltage Vcc and its AC value oscillates around this DC value. In the operational mode, the instantaneous value of the voltage at the associated pin will therefore regularly go beyond the level of the positive supply voltage Vcc. If the device of FIG. 1 for protection against electrostatic discharges is placed on this pin in the operational mode, then this device will get activated. The integrated circuit therefore cannot fulfill its function. The prior art protection device is therefore not compatible with applications of this type.